Most machinery or industrial equipment employ sensors for status monitoring or control. Usually, the machinery is relatively complex and more than one sensor is employed to sense more than one parameter, event, or situation. These sensors are connected to monitoring circuits and together or in combination form a sensor monitoring system. Primary goals for these sensing systems are that they be reliable and inexpensive to apply. Some widely used methods of maximizing reliability are to create sensors with the least amount of components to reduce failure rates, the least amount of interconnections possible to reduce wiring faults, and to provide some form of self-diagnostics to determine the sensor's operational status to separate sensor failure from other system problems. Reducing the number of components and the number of interconnections also serves to reduce the overall cost of applying the sensors. Other methods also exist to reduce the application cost while not reducing reliability. A method of reducing sensor application cost while increasing reliability is to connect the sensors in a multiplexing scheme where possible to use a minimum amount of interconnects and external wiring.
Sensors generally have one of three types of outputs: an analog output proportional to the quantity or relative quality of a sensed parameter or to the position of an object or objects, a switched output that is generated when a sensed parameter or the position of an object or objects ranges outside a preset limit, and a digital output that is usually an encoded signal proportional to the analog measurement of the parameter being sensed or to the position of an object or objects. Four possible applications exist wherein more than one of these sensors is applied that determine whether the sensors can be multiplexed and how they can be so connected.
The first application is for machinery that is structured so all the events are either mechanically interconnected or otherwise controlled so that no event that is being monitored occurs at the same time as any other event. This is known as mutually exclusive event monitoring. A primary example of a type of machine that uses this scheme is an internal combustion engine. On most engines up to eight cylinders, the operation of each cylinder is synchronized to all the others through mechanical linkages in a manner such that only one cylinder is generating power at any time. A system monitoring the status of the fuel injectors in the individual cylinders of these engines would be a mutually exclusive event monitoring system. If an event occurs, the monitoring system can use engine timing information such as the rotational position of the crankshaft to determine in which engine cylinder the event is occurring. All sensors can therefore be multiplexed together with no regard to whether any sensor signal will conflict with the signal from another sensor.
A second situation exists wherein the arrangement of machinery is unstructured so any single event can occur at any time regardless of the status of any other event. This situation is known as real-time event monitoring. In real-time event monitoring, there are two situations in which the sensors are applied. A first situation is delineated by whether the events are variable in nature and contain information that must be decoded. Analog or digital sensors monitor these events. A second situation exists where the information desired is whether any one of a plurality of events is simply beyond a preset limit or exceeds a preset parameter. These events are monitored with switched output sensors.
In a first real-time event monitoring situation, the important information about the events being monitored are the actual details about the event itself, for instance, when and how far any single item moves. This is known as discrete event monitoring. A prime example of this system is the monitoring of individual wheel speed sensors on an automotive antilock brake system. In the case of wheel spin or loss of traction, any or all of the individual wheels can have different speeds. The system must know which wheel has a particular speed to adjust the braking force on that wheel only. Each sensor must be monitored separately to determine that particular wheel's speed.
In a second real-time event monitoring situation the sensor monitoring system simply delivers information relating to the status of a collection of events, for instance, whether all items being monitored are in an appropriate or safe position. This is known as one-of-any alarm monitoring. A primary example of this type system is the monitoring of the position of the doors on an automobile. The system delivers an alarm if any door is open. Information about which door is open, or how far it is open, is not needed to generate the alarm condition. These sensors can be multiplexed together, as long as two conditions exist. The first condition is that the alarm signal is not lost if more than one sensor transmits information at the same time. For instance, in the door open alarm situation, if more than one doors is open, the signals from the sensors should add in value and not cancel each other out. A second condition exists when either the particular sensor with the alarm condition is not delineated to the monitoring system, or when the sensor generates some sort of signal so the monitoring system will know specifically which alarm is being generated. It is a common practice in the one-of-any alarm monitoring systems to not delineate the specific location of the alarm but to simply announce that at least one alarm condition exists, in favor of reduced system complexity. The usual alternative to this least expensive monitoring system is to program each sensor with a specific digital signal that identifies the particular sensor generating the alarm. This method, however, requires that the sensors not transmit information simultaneously.
A further method of multiplexing sensors that generate digital signals is to assign each sensor a unique number that delineates the sensor's position and function. This is called a multiplexed digital sensor system. The sensor transmits these numbers along with a number denoting its status either upon a specific event, on regular intervals, or when polled by the sensor monitoring system. There are three methods of assigning unique numbers to each sensor. In the first method, a unique number is assigned during manufacture of the sensor. This is usually accomplished by individually encoding digital numbers into semiconductor material used to generate the sensor circuitry. A second method is to provide a series of switches that the end user programs to individually assign a unique number to each sensor. A third method is to allow the end user to assign a unique number to each sensor by programming methods. Each method is relatively expensive to employ. Assigning a unique number during manufacture requires complex procedures such as vaporizing metal traces with a laser or burning open diode junctions with high voltages. Employing a series of switches or other programming means for the end user to uniquely assign numbers increases the size and complexity of the sensor, increasing cost and reducing reliability. These methods also require that the end user or monitoring system keep track of the numbers assigned so no two sensors have the same number, and so replacement sensor can be assigned the same number as the sensor being replaced.
It is now recognized and addressed by the present invention that an important distinction exists between these four forms of sensor monitoring as it applies to the wiring of the sensors. Sensors used in the least expensive one-of-any alarm monitoring where the particular sensor generating the alarm is not identified, and sensors used in mutually exclusive event monitoring can have analog outputs and can have their outputs connected to the same point electrically because either the events being monitored can only occur independently of each other, or because the information desired is whether any event among a set of possible events is occurring. The determination of which event is occurring can be derived by some other means such as the timing of related events, or the determination of which event is occurring is not as important as the determination of the fact that some specific event is occurring. The discrete event monitoring system and multiplexed digital sensor system cannot use analog signals connected in this simplest multiplexing scheme without some method of insuring that the information delivered by any one sensor is not superimposed over the information delivered by any other sensor. A common practice for these discrete event monitoring systems is to either connect analog outputs separately at the monitoring system, or to digitize the signals and have the sensors transmit the data separately in time so no sensor signal interferes with any other sensor signal.
It is further recognized and addressed by the present invention that a significant problem in sensor systems is that a sensor may itself fail and indicate falsely that either an alarm condition exists or that an event is or is not occurring, regardless of the true status of the system being monitored. It is imperative in some situations that the sensor monitoring system is capable of diagnosing the sensors to determine if a signaled event or problem is due to system problems or simply due to sensor failure. Knowing that a problem is due to simple sensor failure may mean the difference between being able to run the system in a limited mode or having to shut it down, or in operating dangerously when an alarm condition exists that is not being signaled by a broken sensor.
Most prior art position and proximity sensor devices transmit information in one of three ways. Analog position sensors generate an analog signal proportional to a specific parameter being monitored, such as the temperature or the position of one or more objects. Digital proximity sensors generate one or more digital pulses when one or more events occur such as a temperature going above or below a specific set point, or an object moving beyond a specific position. Digital encoder sensors generate digital information concerning the position of an object, the occurrence of an event, or the status of a situation.
Prior art analog position sensors sense the location of objects by sensing the presence of or the relative amount of specific items or materials within their sensing range. Various examples of the technology employed in these sensors are inductive, capacitive, or magnetic sensors. The analog position sensor output is proportional to the position or movement of the object being monitored. These sensors usually rely on the generation of static electromagnetic or magnetic fields and the subsequent dynamic change in the field caused by the movement of the object being monitored. The static fields generated or otherwise used by these sensors are usually at least an order of magnitude larger than the dynamic change in the field caused by the object's movement. These sensors therefore usually transmit large static DC signal offsets and smaller dynamic signals as an object is monitored. This is because there is usually a significantly larger amount of similar nonmoving material in the sensing range of the sensor. For instance, small moving valves are usually surrounded by large rigid housings. This is especially true of magnetic sensors because they usually rely on large, high strength magnets to generate the static field. It is yet further recognized and addressed by the present invention that there is a significant problem is encountered with these sensors if they generate signals using current modulation. The usually quite large static offset signal produces a correspondingly large current, and the dynamic signal as the target moves is usually quite a bit smaller. The large static current causes remote monitoring swamping resistors to drop a large portion of the applied voltage that is used to power the sensor. Also, the monitoring equipment must be capable of ignoring or eliminating the large static offset and amplifying the smaller dynamic signal.
Prior art pulse output proximity sensors generate one or more digital on/off pulses that change state when an object moves between one or more defined positions or moves toward or away from some preset position. These sensors contain output-switching comparators with preset thresholds and hysteresis. Usually, the sensor must be placed in a specific position in relation to the object being sensed to allow the comparators to switch at a point approximately halfway between the extremes of the object's range of movement. It has also been recognized and addressed by the present invention that unless complex compensation circuitry is added to the sensor, the timing of these output pulses in relation to specific object movement can vary significantly from one sensor to the next or from one event to the next because of tolerances in mechanical and electrical components due to changes in electronic devices in the sensor due to aging or temperature changes. A significant advantage is realized in the application of these type sensors if they are produced with the ability to reprogram specific set points independently of the position of the sensor relative to the target.
A problem with the multiplexing of these switched output sensors is that some method must be employed to allow the monitoring system to determine which sensor is in the alarm state if this information is needed. Usually, this is done with each sensor transmitting a digital number delineating its position or function. This greatly complicates the sensor, and requires that each sensor be given a unique number during manufacture or during initial installation.
Prior art digital encoder sensor outputs also cannot transmit at the same time other sensors are transmitting information. This may be ameliorated somewhat if the information is only transmitted after all other sensors have transmitted their information, usually after the event being monitored is occurring. This usually precludes real-time monitoring of these sensors in a multiplexed arrangement. Connecting these sensors to a single interconnection point requires that some method be employed to prevent them from transmitting information at the same time.
It has been found that semiconductor devices such as the three prior art sensors above are usually very susceptible to output changes due to changes in temperature. Temperature changes can change the gain of the analog position sensor and can change the response level and pulse timing of the digital proximity sensor and digital encoder. Also, extreme temperatures can destroy or damage them. Prior art sensor monitoring systems do not usually contain temperature compensation for extreme changes in sensor output as a result of temperature changes nor do they supply any indication that the temperature is above a dangerous level.
Two-wire sensors are connected via two wires and generate signals by modulating the amount of current passing through a remote resistor connected in series between the sensor and a remote monitoring circuit connected to these two wires. The sensor signal is developed into a voltage signal across the resistor. The voltage across the resistor becomes the signal from the sensor. Two-wire technology is less expensive and more reliable than technologies using more wires. Two-wire connection of the three main type of prior art sensors, however, is made difficult by various problems specific to the type of sensor being employed. The smaller dynamic signal must be amplified more to overcome noise. The analog output position sensor's large DC offset through the monitoring resistor wastes power and reduces effective power supply voltage to all sensors. The proximity sensor and digital encoders generate digital pulses that must be transmitted separately from the pulses from any other sensor and some manner of identifying which sensor is generating the information must be employed.
These sensors may be multiplexed, or a multiplicity of sensors can be connected in parallel to the same set of two wires, as long as the following criteria are met:
First, in the case of mutually exclusive event monitoring, mechanical or electrical limitations on the system preclude any of the monitored events from occurring at the same time. In this case we usually know which event should be occurring at any one time and know therefore that the sensor outputting a signal has to be monitoring an event of particular interest. A prime example is the monitoring of the operation of fuel injectors on internal combustion engines. The engine controller or fuel pump may initiate the injections. We know when a particular injection from a particular injector should occur, within a window of time. The sensors can be made very inexpensively because complex circuitry is not required to prevent the sensors from generating conflicting information at the same time. These sensors can generate analog signals, alarm pulses, or digital information without generating a signal at the same time as any other sensor.
Second, the situation may be such that we only want to know that all the sensors are in a particular desired situation. The sensor only transmits information if the situation changes at any one of the monitored stations. It does not matter which sensor has the problem, or that more than one sensor has an alarm situation. This is known as monitoring one-of-any alarms. A prime example is a network of door sensors.
Accordingly, there is a need for a sensor monitoring system that overcomes the limitations of prior art sensors and monitoring systems when monitoring mutually exclusive events or one-of-any alarm conditions.